I would start with a PRAM reset. Whenever you are adding new hardware to any Mac perform a PRAM reset. Reboot and hold the Option, Command, P, and R keys simultaneously...do this until you hear the Start Up chime twice, then release the keys. This failing, try to boot into Safe Mode by holding the shift key during bootup. Safe mode boot only loads the necessary drivers and also allows the system to correct Drive Directory issues, and performs tests and fixes system and file errors that can prevent a proper boot. If you successfully boot inyo safe mode, run disk utility and verify and make any necessary repairs to the drive or permissions and reboot normally soon after.

My ibook G4 is having the same issue. Tried resetting PRAM- no good. Tried shift key and got kernel info across the screen w/the end message being: "we're just hanging out here" Is the HD dead? Sometimes it shows up in Disk Utility w/no size or specs, sometimes not at all. Any other ideas?